1. Field of the Invention
The present invention relates to a drive circuit of an active matrix type organic EL panel and an organic EL display device using the drive circuit and, in particular, the present invention relates to an active matrix type organic EL display device, which can reduce luminance unevenness of a display screen of a portable telephone set or PHS, etc., even when the size of pixel circuit is restricted by removing a circuit for compensating for an operating threshold value of drive transistors and is suitable for high luminance color display.
2. Description of the Prior Art
It has been known that an organic EL display device, which realizes a high luminance display by spontaneous light emission, is suitable for a display on a small display screen and the organic EL display device has been attracting public attention as the next generation display device to be mounted on a portable telephone set, a PHS, a DVD player or a PDA (Personal Digital Assistants), etc. Known problems of such organic EL display device are that, since, when it is driven by voltage as in a liquid crystal display device, luminance variation thereof becomes substantial and that, since there is difference in sensitivity of organic EL element between R (red), G (green) and B (blue), a control of luminance of a color display becomes difficult.
In view of these problems, an organic EL display device using current drive circuits has been proposed recently. For example, JPH10-112391A discloses a technique in which the luminance variation problem is solved by employing a current drive system.
An organic EL display panel of an organic EL display device for a portable telephone set, a PHS, etc., having 396 (=132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed. However, there is a tendency that the number of column lines as well as row lines is further increased.
An output stage of a current drive circuit of such organic EL display panel of either the active matrix type or the passive matrix type includes a current source drive circuit, such as an output circuit constructed with a current mirror circuit, for each of the terminal pins.
In the active matrix type organic EL display device, a pixel circuit composed of a capacitor and a current drive transistor is provided for each display pixel. The transistor of the pixel circuit is driven by a voltage stored in the capacitor and the organic EL element (referred to as OEL element, hereinafter) is current-driven by the transistor. A drive system therefor is a digital drive system in which the OEL element is binary-controlled by ON/OFF of the transistor or an analog drive system in which a drive current of the OEL element is controlled by an analog input data. In the case of the digital drive, a display area is controlled by providing a sub pixel in the pixel or a color tone of a display pixel is controlled according to a difference of a drive time by time-dividing a light emitting time. The analog drive system is classified to a voltage assigning type system (voltage program system) and a current assigning type system (current program system). In the voltage assigning type analog drive system, a terminal voltage of the capacitor of each pixel circuit is set by a voltage signal and, in the current assigning type analog drive system, the terminal voltage of the capacitor is set by a current signal.
In such active matrix type organic EL display device, luminance unevenness tends to occur due to variation of operating threshold value of the drive transistor in each of the pixel circuits. Since it is difficult to make the operating thresholds of the drive transistors of the respective pixel circuit uniform during a fabrication process of the display device, it has been considered to restrict the luminance unevenness by controlling the voltages of the capacitors of the respective pixel circuits. In order to realize such scheme, a threshold value compensation circuit is provided within the pixel circuit. Examples of the threshold value compensation circuit are of the voltage program type and the current program type.
The voltage program type threshold value compensation circuit utilizes four transistors and two capacitors provided in each pixel circuit and two wiring lines for compensating for variation of the operating threshold value of the drive transistors are provided in addition to data lines and selection lines. The current drive is performed without influence of the threshold values of the drive transistors by charging the two capacitors by control signals to the two lines with predetermined timing.
In the current program system, the pixel circuit includes three transistors, which includes the drive transistor, and a switching transistor for setting a specific voltage. Further, the pixel circuit includes a data line, two selection lines and a power source line (source line) of a specific voltage Vdd. First, the drive transistor is separated by the switching transistor to charge the capacitor by current drive and then the drive transistor is connected to the capacitor by the switching transistor and the OEL element is current-driven by supplying power from the source line.
Each of these drive methods requires a program timing control. Particularly, in the current drive system, the preciseness of current value required for intermediate tone display control becomes 0.1 μA or smaller. Therefore, the control itself becomes difficult. Further, when the density of display pixels becomes as high as, for example, VGA, SVGA, XGA, etc., the circuit size of each pixel circuit becomes large since the program timing control must be performed within a limited time and lines for the program control are required in addition to the data line and the selection line.